This invention is generally related to machine-readable symbol readers.
In known optoelectronic devices for acquiring machine-readable symbols, such as bar codes, a diaphragm has a circular aperture of small diameter so as to prevent defocusing of an image and/or to increase the depth of field of the device. The small diameter of the aperture, however, reduces the intensity of reflected light received at the sensor and, in practice, makes it necessary to use light sources having a high luminous intensity in order to compensate for the reduction in luminous intensity introduced by the aperture. However, high intensity light sources are expensive and lead to high power consumption.
While increasing the diameter of the aperture of the diaphragm increases the quantity of light received by the sensor, the increase in diameter also reduces the depth of field of the device, thereby reducing the overall efficiency of the device.
One attempt at solving these problems involves producing an optoelectronic device as described in patent application EP-61000, where the diaphragm has an aperture having an asymmetrical elongation along an axis orthogonal to the axis of the bar code, such as an aperture of rectangular, rhombic or elliptical shape. This effectively increases the sensitivity of optoelectronic devices, which is proportional to the ratio of collected flux to reflected flux. As a result, the depth of field of these devices may be increased without significantly affecting the intensity collected on the sensor, thereby increasing the efficiency of these devices. The relatively large dimensions of the diaphragm aperture, however, makes it necessary to use an asymmetrical diaphragm, and optical means for forming the image on the sensor having dimensions greater than those of conventional optical means, which increases production costs and complexity greater than those of conventional diaphragms and optical means.
Another attempt at solving these problems involves producing optoelectronic devices as described in International Patent Applications WO-9620454 and WO-9847377, where the optical means are adapted to obtain, in the plane (XOZ) parallel to the optical plane, a magnification ml greater than the magnification m2 in the plane (YOZ) perpendicular to the optical plane.
This approach, which can also be associated with that described in the patent EP-61000, leads to an increase, along axes parallel to the bars of the bar codes, in the size of the illumination surface of the bar codes whose image is reflected on the sensor, and therefore to an increase in the sensitivity of the optoelectronic device. It should be noted, furthermore, that since this increase in the sensitivity of the device results from the mere design of the optical means and not from the dimensions of the diaphragm aperture, a device of this type may be equipped with a conventional diaphragm with a circular aperture of small dimensions and therefore with low-cost optical means of conventional dimensions which is easy to produce.
With all these devices in which the diaphragm and/or the optical means do not form a symmetrical system generated by revolution round the optical axis, the improvement in sensitivity is effective only when the optical plane (plane containing the optical axis and the scanning direction) coincides exactly with the nominal direction of reading of the bar code (perpendicular to the code bars and spaces). Now, as the bar code and/or the optoelectronic device in practice have unfixed orientations in space, this condition is rarely fulfilled. Thus, a device of this type is extremely sensitive to alignment errors between the optical plane and the normal direction of reading and is therefore difficult to handle.
More generally, known optoelectronic devices for acquiring machine-readable symbols can be configured for predetermined characteristics of the symbols to be acquired and/or for predetermined positioning relative to the device. However, these optoelectronic devices have inferior performance if the symbol does not have these expected characteristics or if the positioning is not perfect. As a result, they suffer from a significant reading failure rate, in particular in the case of plurimonodimensional symbols such as PDF 417 codes.
U.S. Pat. No. 5,654,533 describes a two-dimensional symbol reader comprising a two-dimensional sensor and an automatic diaphragm of which the diameter varies to allow appropriate illumination of the sensor. This device does not attempt, and cannot solve the above mentioned problem since, with this device, correct illumination of the sensor corresponds to a diaphragm that produces an inadequate depth of field. Furthermore, this device is limited to the acquisition of bi-dimensional symbols by imagery, in other words by obtaining and analyzing two-dimensional images.
WO-98.16896 describes a two-dimensional symbol reader comprising both an electronic scanning device having a two-dimensional sensor and a laser scanner device. This mixed reader enables the user to select one of the two devices depending on the symbol to be read. It is however very complex and therefore expensive, fragile and awkward to use. In particular, the embodiments disclosed herein avoid the use of laser devices incorporating moving parts.
At present, therefore, there is no optoelectronic device for acquiring machine-readable symbols, such as bar codes, with electronic scanning which has satisfactory performance, particularly in depth of field, which allows the acquisition of symbols with any characteristics which may be not be known in advance. For example, bar dimensions, bar contrast, type of codes, monodimensional or plurimonodimensional codes (in other words formed by a plurality of monodimensional bar codes) such as the PDF 417 codes, or two-dimensional codes, etc.
In one aspect, an optoelectronic device is capable of acquiring bichromatic machine-readable symbols, such as bar codes, formed from monochromatic elements of geometric patterns (e.g., bars, squares, hexagons) having one of two levels of contrasting colors of which the shapes and disposition are adapted so that each code is able to represent bi-uniquely a value of information to be acquired.
In another aspect, a process allows an optoelectronic device to acquire machine-readable symbols based on symbol characteristics. In one aspect, a device and a process acquires machine-readable symbols having different characteristics, in particular of different types, and which may be adapted at the moment of acquisition, in particular automatically, to the characteristics, in particular to the type of symbol to be acquired.
In another aspect, an optoelectronic device and process acquires machine-readable symbols with electronic scanning, while simultaneously providing large depth of field and low rotational sensitivity to alignment errors between the optical plane and the nominal direction of reading of the symbol, without necessitating the use of high intensity light sources.
In yet another aspect, an optoelectronic device does not require a high degree of precision in positioning of the machine-readable symbols to be acquired relative to the device in the relative spacing and rotational alignment around the optical axis, and allow manual acquisition (in other words by relative manual positioning of the device and/or the symbol) of the symbols.
In a further aspect, an optoelectronic device and a process manually acquires (by manual relative positioning of the symbol and/or optoelectronic device) machine-readable symbols from relatively new symbologies such as PDF 417 codes.
In yet a further aspect, an optoelectronic device can provide the above benefits while being inexpensively manufactured in a traditional manner, which requires no moving parts.
In still a further aspect, acquiring symbols can be performed by a simple and quick process, which can be entirely automated.
To this end, a non-limiting, illustrated embodiment of an optoelectronic device for acquiring bichromatic bar codes, comprises:
a reading window,
sensor means with electronic scanting comprising a two-dimensional sensor comprising a plurality of individual detectors known as pixels transmitting electrical signals representing the quantity of light which they receive, the sensor means being adapted to carry out electronic scanning or at least a portion, known as scanned portion, of this two-dimensional sensor in a direction, known as scanning direction XXxe2x80x2, the pixels of the two-dimensional sensor being ordered in a plurality of h rows juxtaposed in a direction, known as direction YYxe2x80x2, perpendicular to the scanning direction XXxe2x80x2, the two-dimensional sensor/extending in the direction YYxe2x80x2 over a height greater than a pixel, the scanned portion having a dimension in the direction YYxe2x80x2, known as height Hy, which is constant during each scanning operation, from one side to the other of the two-dimensional sensor in the scanning direction XXxe2x80x2,
optical means adapted to form, at least on the scanned portion of the two-dimensional sensor, an image of a symbol or code to be acquired located opposite the reading window, wherein, in order to acquire a code placed opposite the reading window, the sensor means are adapted to carry out at least two scanning operations (i.e., passes) and to modify, between at least two successive scanning operations, the height Hy of the scanned portion of the two-dimensional sensor.
Throughout the text, the term xe2x80x9crowxe2x80x9d denotes each series of successive individual pixels which can be covered pixel by pixel during a scanning operation in the scanning direction. A row is therefore defined by the geometric arrangement of the pixels of the sensor in the scanning direction XXxe2x80x2 and by the way in which these pixels considered individually are covered during the scanning operation. In the simplest case of pixels arranged in lines and scanning carried out over each line, a row corresponds to a line. However, if the pixels of two adjacent lines are alternated during the scanning operation, a row is thus formed by the pixels of these two lines. While scanning along a row in the scanning direction XXxe2x80x2, electrical signals are received from one or more pixels arranged across the rows with respect to one another in the direction YYxe2x80x2, perpendicular to the scanning direction XXxe2x80x2.
Also to this end, a non-limiting, illustrated embodiment of a method of operating an optoelectronic device for acquiring bichromatic symbols, comprises:
a reading window,
sensor means with electronic scanning in a global scanning direction, known as scanning direction XXxe2x80x2 comprising a plurality of individual light detectors known as pixels transmitting electrical signals representing the quantity of light which they receive, these sensor means comprising a two-dimensional sensor of which the pixels are ordered in a plurality of h rows juxtaposed in a direction, known as direction YYxe2x80x2, perpendicular to the scanning direction XXxe2x80x2, this two-dimensional sensor extending perpendicularly to the scanning direction XXxe2x80x2 over a height greater than a pixel, the sensor means being adapted to carry out electronic scanning of at least a portion, known as scanned portion, of the two-dimensional sensor having a dimension in the direction YYxe2x80x2, known as height Hy, which is constant during each scanning operation, from one side to the other of the two-dimensional sensor in the scanning direction XXxe2x80x2,
optical means adapted to form, on the sensor means, an image of a symbol or code to be acquired located opposite the reading window,
a process for acquiring bichromatic bar codes, wherein, in order to acquire a symbol or code placed opposite the reading window, at least two scanning operations are carried out and, between at least two successive scanning operations, the height Hy of the scanned portion of the two-dimensional sensor is modified.
In a device and a process according to the invention, the height Hy can be modified once; or several times but not between the scanning operations each time; or between two successive scanning operations each time in order to acquire the same symbol or code.
To modify the height Hy of the scanned portion, it is possible to modify either the height of at least one row of the scanned portion (by selecting a row of which the pixels have a different height pyj) or the number of rows in this scanned portion, in other words the number of successive pixels in the direction YYxe2x80x2 of which the signals are added up in a same signal used during the decoding operation. These two variations may be combined. It is in fact possible to modify both the number of rows and the height of at least one row. In fact, the height Hy of the scanned portion is equal to the sum of heights pyj of each row j of this scanned portion. If all the heights pyj of the rows are equal to a same value py and if the scanned portion comprises by rows, the height Hy of this scanned portion is equal to hy x py. If the rows do not all have the same height pyj,   Hy  =                              ∑                      xe2x80x83                    ⁢          pyyj                hy                    j        =        1              .  
In a variation, therefore, the device according to the invention is characterized in that each row is formed by pixels all having the same dimension in this row in direction YYxe2x80x2, known as height pyj, wherein the pixel height pyj of at least one row of the two-dimensional sensor is different from that of the pixels of at least one other row of the two-dimensional sensor and wherein, in order to modify the height Hy of the scanned portion, the sensor means are adapted to carry out at least one scanning operation, known as first scanning operation, with at least one row of pixels and at least one further scanning operation, known as second scanning operation, with at least one row having a pixel height pyj different from that of at least one row of the first scanning operation. Advantageously and according to the invention, the sensor means are adapted to carry out at least one second scanning operation with at least one row having a pixel height pyj different from that of each row of at least one first scanning operation.
In a further variation of the invention, in order to modify the height Hy of the scanned portion, the sensor means are adapted to modify the number, known as pitch try, of successive rows of the scanned portion.
The scanned portion of the sensor is the one comprising the pixels of which the signals are used to decode a symbol or code on the basis of a scanning operation. By modifying the value of the height Hy or the portion scanned between at least two scanning operations, the device adapts itself or may be adapted to the type and/or to the characteristics (which may be unknown) of the symbol or code to be acquired since at least some of the different values used for the height Hy will be most suitable.
In a first variation of the invention, the various possible values of the height Hy may be predetermined in advance (for example if the variations in height Hy are obtained by selecting rows from a plurality of different heights pyj) and optionally stored (for example various predetermined values for the number hy of rows) in the device comprising electronic processing means adapted subsequently to select the best result obtained by the various scanning operations in order to execute a decoding protocol. In particular, this variation is applicable if the type of symbol is known but not the optical characteristics of the codes to be read (contrast, dimensions, etc.).
In a second preferred variation of the invention, the device automatically adapts itself to the codes to he read, of which the type and characteristics may be unknown. Advantageously, the device according to the invention comprising electronic processing means adapted, during each reading of a symbol or code to be acquired:
to control the scanning operations by the sensor in the scanning direction XXxe2x80x2 and receive the electrical signals issuing from the pixels,
to execute a predetermined decoding protocol in order to obtain the value of information represented by the symbol or code,
wherein the sensor means are adapted to, after each scanning operation, execute treatment to optimize the height Hy in order to improve the results of the subsequent scanning stage and reduce the number of scanning stages required for decoding, wherein, during this optimization treatment, an optimized value of the height Hy which is to be used during a subsequent scanning operation is determined as a function:
of at least one previously measured value of at least one parameter representing the quality of the image acquired by the sensor means,
and/or of at least one item of information issuing from a previously executed decoding stage,
and wherein the sensor means are adapted to record the optimized value of the height Hy determined in this way to be used during a subsequent scanning operation.
In a device and a process according to this second variation of the invention, the value of the height Hy, in particular the pitch hy and/or the selection of the row(s) of height pyj used, is therefore optimized after each scanning operation to improve the results of the subsequent scanning stage thus enabling the decoding process to be executed and accelerated and enabling the number of scanning stages required for decoding to be reduced, with a field depth, electricity consumption and rotational sensitivity round the optical axis which are compatible with practical use of the device, and with an electronic scanning device which is free from moving parts.
In particular in the case of simple bar codes or bar codes of the PDF 417 type, the electronic processing means determine, after each scanning operation, the value of the height Hy optimized to obtain the best field depth with given rotational sensitivity.
Advantageously and according to the invention, the optimized value of the height Hy is determined by computation, by closed loop control on the basis of a reference value of a parameter or by optimization control by comparing the evolution of at least one parameter of a reading operation to another. Advantageously, therefore, automatic control is incorporated in the electronic processing means.
Advantageously and according to the invention, the optimized value of the height Hy is determined as a function of at least one previously measured value, in particular after the previously effected last stage of scanning, of at least one parameter representing the quality of the image acquired by the sensor means selected from the maximum spatial frequency fx of the symbol in the scanning direction XXxe2x80x2, the maximum intensity of at least one category of symbol image elements, the minimum intensity of at least one category of symbol image elements and the contrast of at least one category of symbol image elements. Further similar parameters may be used as an alternative or in combination.
Advantageously and according to the invention, the optimized value of the height Hy is determined as a function of at least one item of information relating to the type of symbol to be acquired and issuing from a previously executed decoding stage.
Advantageously and according to the invention, each previously measured value and/or each item of information used to determine said optimized value(s) has been obtained and recorded during a scanning operation immediately preceding said optimization treatment.
Advantageously and according to the invention, the electronic processing means are adapted to fix by default and to record an initial value Hyxc2x0 of the height Hy before a first scanning operation in order to acquire a code and/or after a last scanning operation in order to acquire a symbol or code, in particular a value hyxc2x0 of the pitch hy. For example, hyxc2x0-h/2 may be selected in which h is the total number of rows corresponding to the total height H of the sensor in direction YYxe2x80x2.
Advantageously and according to the invention, the electronic processing means are adapted to, after each scanning operation:
determine the measured value of the maximum spatial frequency fx of the symbol image in the scanning direction XXxe2x80x2,
calculate and record the optimized value of the height Hy on the basis of an affme function of the inverse of the measured value of the maximum spatial frequency fx of the code image in the scanning direction XXxe2x80x2.
Advantageously and according to the invention, moreover, the electronic processing means are adapted to determine the optimized value of the height Hy according to functions parameterized by predefined values, in particular predefined by the user or during manufacture and stored in a read-only memory of the device, of parametric coefficients linked to the type(s) of symbols to be acquired.
Advantageously and according to the invention, the electronic processing means are adapted to determine, after at least one scanning operation, in particular after a first scanning operation in order to acquire a symbol or after each scanning operation, the corresponding type of symbol and the value of the corresponding parametric coefficients. For example, if a number of gray levels higher than 2 is detected with a characteristic homogeneous spatial frequency, it is probable that the symbol is of the PDF 417 symbology type and therefore comprises a plurality of bar fmes and that Hy was greater than the height of the image of a line of bars of the symbol. It is thus possible to impose subsequent criteria on the height Hy, in particular on the pitch try, in particular that Hy is smaller than 4 times the width of the image of the finest element of the symbol, this being a necessary condition for acquiring PDF 417 symbols.
Advantageously and according to the invention, the electronic processing means are adapted to calculate the optimized value of the height Hy according to a function parameterized by a predefined value of the maximum permitted angular deviation xcex8max of the sensor round the optical axis ZZxe2x80x2 relative to the symbol to be acquired.
In an advantageous variation of the invention, the electronic processing means are adapted to determine and, if necessary, modify the optimized value of the height Hy in order to optimize the measured value of the contrast of at least one category of symbol image elements. To this end, the electronic processing means may include closed loop control adapted to optimize the contrast.
In a further variation of the invention, the optimized value of the height Hy is determined by computation.
Advantageously and according to the invention, the two-dimensional sensor is a surface sensor formed by a CCD or APS matrix of pixels. This sensor can have a plurality of embodiments.
In a first embodiment, the pixels in the same row are juxtaposed and are aligned in the scanning direction XXxe2x80x2, the sensor being formed by a pixel matrix having h lines. In other words, each row of pixels is formed by one of the lines of the sensor. The pixels are generally square or rectangular.
In further embodiments, the electronic processing means and the sensor are adapted so that the pixels in the same row belong to two distinct lines of sensor pixels which are adjacent to one another in direction YYxe2x80x2, parallel to the scanning direction XXxe2x80x2, the successive pixels in each row, when covering a row in the pixel-by-pixel scanning direction, alternately belonging to either of these two lines. This variation enable the dimension of the sensor in the scanning direction text to be reduced with the same resolution. The pixels may be square or rectangular, or may have other, generally polygonal, shapes.
Furthermore, the height ply of the pixels in each row may be the same (py) in all rows or, on the contrary, different from one row to another.
Advantageously and according to the invention, moreover, the optical means comprise a diaphragm of which the dimensions are approximately the minimum dimensions corresponding to the theoretical limit of diffraction and are always greater than these minimum dimensions. Owing to the invention, the smallest diaphragm allowed by the theoretical limit of diffraction may in fact be adopted. Advantageously and according to the invention, therefore, the dimension li of the diaphragm in the direction IIxe2x80x2 selected from the scanning direction XXxe2x80x2 or the direction YYxe2x80x2 is roughly but greater than:
xcexaxcexf(l+mi)/pi.Nmini
wherein
xcex is the wavelength of the lighting means,
f is the focal length of the optical means,
mi is the magnification of the optical means in the direction IIxe2x80x2,
pi is the dimension of the pixels of the sensor means in the direction IIxe2x80x2,
Nmini is the minimum number of successive pixels the direction IIxe2x80x2 which have to be contained in the image of a code element on the sensor to allow the decoding thereof,
k is a form factor of the diaphragm.
Advantageously and according to the invention, an optimization treatment characterized by at least one of the characteristics described above in relation to the device according to the invention is executed. The invention therefore also relates to a process according to one of claims 18 to 32.
It should be noted that U.S. Pat. No. 5,319,182 describer a mixed sensor which mixes light-emitting elements and light-sensitive elements which can be used in a bar code reader comprising a matrix of emitting and receiving diodes, and aims to provide axial lighting for the target aligned with the field of vision on the target to avoid the effects of diffusion and of layers. This document mentions configuration and optimization of the grouping and proportion of emitters and detectors in the matrix according to the image processing application thereof, in particular for the reading of bar code symbols. However, this document does not describe an electronic scanning device capable of adapting itself to the unknown characteristics of a symbol to be read and in which the number of pixels in the direction perpendicular to the scanning direction is modified between two successive scanning operations.
The invention also relates to a device and a process which are characterized in combination by all or some of the characteristics mentioned hereinbefore or hereinafter.
Further objects, characteristics and advantages of the invention will emerge on reading the following description which refers to the accompanying figures describing various non-limiting embodiments of the invention.